Reinforced wood particle board and a method of producing it

ABSTRACT

The invention relates to particle boards, e.g. wood particle- or chipboards, for building purposes, which have been reinforced by means of glass fibre filaments or threads placed in the boards. The threads are laid to obtain increased rupture strength and increased resistance to punch-through of concentrated loads, and are fixed at given spacing in conjunction with the board manufacturing process. The method of reinforcing the boards during manufacture is also included by the invention.

The present invention relates to the reinforcement of wood particleboards by means of reinforcing filaments of threads suitably fixedtherein, and a method of reinforcing wood particle boards, e.g. for useas roofing boards directly against roofing trusses. The characterizingfeatures of these reinforced wood particle boards and the method used toproduce them will be seen from the following description and claims.

Different methods of reinforcing wood particle boards are already known,e.g. from the Swedish published specifications Nos. 7600758-2 and7612014-6. According to these known methods, a reinforcing layer of netor unbleached paper having high tensile strength is used. These layersare glued onto the top or bottom faces of the boards in a separateprocess after manufacturing the boards. Special production lines areused for this process, in which the manufactured and finished boards arereinforced in order to obtain properties increasing their strength.

These known methods all have the drawback that the movement or creep ofthe reinforcing material, due to the effects of moisture and heat,deviates from that of the board material itself. As a result of thereinforcing material being applied to one side of the board, a so-called"bimetal" effect occurs, manifesting itself by the board becomingwarped. The thinner the board is, the greater is this deformation,partly for reasons of geometry, and partly because thinner boards have,per se, less resistance to bending. This effect is apparent when theboards are painted, for example, a thin coat of paint being sufficientto cause deformation.

It has now been found that a very high rupture strength can be obtainedfor the wood particle board if, in accordance with the presentinvention, it is provided with interior reinforcement comprising glassfibre filaments or threads in conjunction with the manufacturing processfor it. These threads, preferably gathered into clusters or bundles, arelaid parallel, with constant or varying mutual spacing and are advancedcontinuously from bobbins onto a continuously advancing bottom web ofthe fibre of the fibre stock from which the board is formed, synchronouswith the formation of the wet fibrous web, but before applying the topweb of fibrous stock forming the whole of the still unfinished board.During subsequent dewatering and pressing steps, the threads are thusembedded and fixed between the top and bottom webs of material formingthe board. Reinforcement over a larger surface area than would otherwisebe obtained may be provided by laying the threads in a wavy andsinusoidal pattern.

The spacing of the threads to suit requirements relating to rupturestrength as well as to the manufacturing process and cost of materialhas also been found to have great importance. While too large a distancebetween the threads or bunches thereof does not provide the desiredreinforcing effect on the one hand, very small spacing gives raise tosuch practical drawbacks as excessive weight and material cost on theother hand. Furthermore, it has been found that if thread spacing isreduced to 10 mm or less, dewatering problems which cannot be ignoredoccur in the pressing step of the manufacturing process. Too closelylaid reinforcing threads will thus delay the departure of water, and insome cases there may even be certain bursting effects in the board whenpressure is released.

Although the kind of board dealt with here can take up quite largestresses in the direction of its surface, it is susceptible to loadsperpendicular thereto, particularly concentrated loads. Thus, in manyuses of the boards, such as for roofing or false sealings, there isalways the risk of a punch-through, i.e. local rupture or collapse ofthe board due to a concentrated load, e.g. such as is caused by theweight of a person, or dropping a heavy tool.

It has thus been found that suitable spacing of the threads (measuredbetween the centre lines of adjacent threads or clusters of filaments)should not fall below 10 mm. On the other hand, to avoidpunching-through and to maintain rupture strength, this distance shouldnot exceed 80-100 mm. In practice, a spacing (as just defined) between aminimum of about 20 mm and a maximum of about 60 mm should be selected,the selection being made to given an optimum distance in respect ofpertinent rupture strength demands and extra cost of reinforcemenet. Aspacing of about 40 mm has accordingly been found to be suitable forboards thus reinforced in accordance with the invention, for use asfalse ceiling boards.

Reinforcement at the edges of the board is of great benefit to nailingstrength, and since location of the reinforcing threads is optional,they may be placed in the vicinity of the edges in the rollingdirection. This advantage is particularly noticeable with said edgeswhich are given a lower volumetric weight than the rest of the materialduring the pressing operation during manufacture, thus giving thematerial deteriorated strength along a margin of about 2-3 cm, which istroublesome in conventional boards. Thus, with reinforcement in thevicinity of the edges of the board, the tendency thereof to tear awayfrom the shank of the nail for a large load if the nail is to near theedge is obviated, as well as substantially reducing the tendency of thenail to pull through the board axially to the nail for an adjacentconcentrated load urging the board away from its substructure.Accordingly, the problems of attaching the board to a substructure arethus greatly reduced or solved by the inventive reinforcing method withreinforcing material embedded in, and adhered to the wood particlematerial of the board. Since nails can be driven closer to the edges ofthe board without the risks just mentioned, the overlap of the boards atthe trusses can be reduced to a minimum, which results in maximumutilization of board area. Reinforcement in accordance with theinvention also has the advantage that when it is used, e.g. inconnection with boards for false ceilings, about 50% of the wood fibrenormally used can be saved, since board thickness may be reduced from4.5 mm for conventional boards without reinforcement to 3 mm forreinforced boards. This saving of 1/3 of the material corresponds toabout 10,000 tonnes of wood fibre particle per year in Sweden at thepresent rate of housing production.

To ensure that the glass fibre threads are well fixed in the boardmaterial, the threads are suitably treated with a substance such aslatex, resins, polyvinyl acetate (PVA) or the like, which can bethermosetting, before or in conjunction with embedding in the boardmaterial. A PVA adhesive in an aqueous emulsion and with a somewhatincreased water content (about 40-60%) has given a very good bondingeffect for the purpose. Roughing-up the glass fibre threads can alsoprovide increased adherence.

In laboratory experiments for comparing the ultimate strength ofconventional boards without reinforcement with the same kinds providedwith such in accordance with the invention, the following results havebeen obtained:

    ______________________________________                                              Board                Adhesive                                                                              Relative                                   Sample                                                                              thickness                                                                              Reinforcement                                                                             treatment                                                                             ult. strength                              ______________________________________                                        A.    3.2 mm   None        None    102                                        B.    3.1 mm   None        None     98                                        C.    3.0 mm   Glass fibre,                                                                              None    104                                                       untreated                                                      D.    3.0 mm   Glass fibre,                                                                              None    127                                                       R 1410                                                         E.    3.0 mm   Glass fibre,                                                                              None    121                                                       RPA 38                                                         F.    3.1 mm   Glass fibre,                                                                              PVA glue                                                                              139                                        G.    3.1 mm   Glass fibre,                                                                              PVA glue                                                                              142                                        H.    3.1 mm   Glass fibre,                                                                              PVA glue                                                                              150                                        I.    3.1 mm   Glass fibre,                                                                              PVA glue                                                                              138                                        ______________________________________                                    

The ultimate strength given in the table is a relative number, where 100has been taken as the relative number for the mean value of the ultimatestrength of conventional 3 mm boards with no reinforcement. In theexperiments under discussion, this value represents a pressure of just150 kp/dm² for achieving a rupture of simulated punch-through. As willbe seen, inventive reinforcing of a board with the same thickness asthose without reinforcement results in up to a 50 percent increase inultimate strength. The results which are clearly the best are those fromboards having reinforcement from glass fibre treated with adhesive,which in the experiments accounted for was a water-emulsified PVAadhesive with about 55% water content.

Since security against rupture and punching-through is the primaryreason for reinforcement, it is a great advantage that the latter isplaced in the board and cannot be damaged or become detached from theboard in some way, due to dampness or handling on the building site. Thechoice of glass fibre as reinforcing material has also been made withregard to the fact that this material is resistent to moisture andcorrosion, and also because it has a modulus of elasticity (600 k-1,2Mkp/cm²) desirably high enough for the purpose. The lack of one or moreof the mentioned properties make the use of such threads or filamentsfrom metals; textiles or plastics less suitable as reinforcing material.

The practical utility of the invention is illustrated by the fact thatthe Regulation by the Swedish Board of Occupational Safety and Healthconcerning security against punching-through (Notification 75:15) iscomplied with generously, by using inventive glass fibre reinforcementin hard wood particle board only 3,2 mm thick. A thickness of 4,5 mm forboards without reinforcement is usually required for attaining the sameresult.

The invention can also be utilized in different types of wood particleboard, such as building board and chipboard, where the glass fibrethreads are placed in the neutral plane (median plane) of the boards,thereby providing a stiffening effect in respect of bending. This can beparticularly valuable for boards used in flooring and shelving, sincethey can then be made thinner without sacrificing requirements for formstability on being loaded.

The conventional method of first using threads to make netting to formthe reinforcing layer is an expensive one, since the netting weavingoperation costs about three times more than the original filament orthread reinforcing material. By placing the threads in the wet stocklayer there is avoided the disadvantage if first needing to manufacturethe netting web. Furthermore, by laying the unbroken thread or filamentswithout interruption, improved punch-through safety is obtained in asfar as the dangerous concentrated loads are taken up over a much greaterarea than for surface reinforcement, which could only be bondeddiscontinuously to a board. When thin wood particle board collapses forconcentrated load, this takes place in a sudden and irregular manner.The board does not rupture along straight lines. The new method ofreinforcement therefore signifies that when an area of the boardsuddenly ruptures under large sudden load, a plurality of reinforcingthreads on either side of the load contribute in arresting apunch-through. The same effect as would be achieved with netting is thusachieved to a certain extent, even though the threads or filaments arelaid continuously and parallel in the pulp web.

I claim:
 1. A method of producing a wet laid continuous glass fiberthread reinforced wood particle board comprising, coating the glassfiber threads with an adhesive, dewatering an aqueous wood particlestock to form a first wet layer of wood particles, continuously applyingsaid coated glass fiber threads onto said first wet layer of woodparticles at a side to side spacing of between about 10 mm to about 100mm, applying a second wet layer of wood particle stock on said coatedglass fiber threads and first wood particle layer whereby said coatedglass fiber threads are embedded and fixed between said first and secondwood particle layer pressing to form said board.
 2. A relatively stiffwood particle board produced by the method of claim
 1. 3. A board asclaimed in claim 1, characterized in that the threads comprise clustersor bundles of glass fibre filaments having a modulus of elasticityexceeding 600.000 kp/cm².
 4. A board as claimed in claim 1 or 2,characterized in that the centre to centre distance between adjacentglass fibre threads is not less than 20 mm.
 5. A board as claimed inclaim 1 or 2, characterized in that the centre to centre distancebetween adjacent glass fibre threads does not exceed 60 mm.
 6. A boardas claimed in claim 1, characterized in that the adhesive comprises aPVA emulsion adhesive with a water content of 40-60% at the time of itsapplication.
 7. A board as claimed in claim 1, characterized in that thereinforcing threads are disposed over the entire surface of the board.8. A board as claimed in claim 1, characterized in that the reinforcingthreads are disposed substantially solely at selected edge portions ofthe board.
 9. A board as claimed in claim 1, characterized in that thereinforcing threads are situated closer to the medial plane of the boardthan to the major surfaces thereof, thereby smoothing out possibledifferences in the expansion properties of the board material andreinforcing threads.
 10. A board as claimed in claim 3, characterized inthat the adhesive comprises a PVA emulsion adhesive with a water contentof 40-60% at the time of its application.
 11. A board as claimed inclaim 3, characterized in that the reinforcing threads are disposed overthe entire surface of the board.
 12. A board as claimed in claim 3,characterized in that the reinforcing threads are disposed substantiallysolely at selected edge portions of the board.
 13. A board as claimed inclaim 3, characterized in that the reinforcing threads are situatedcloser to the medial plane of the board than to the major surfacesthereof, thereby smoothing out possible differences in the expansionproperties of the board material and reinforcing threads.
 14. A board asclaimed in claim 1 characterized in that the centre to centre distancebetween adjacent glass fibre threads is in the range of 20-60 mm.
 15. Aboard as claimed in claim 14, characterized in that the threads compriseclusters or bundles of glass fibre filaments having a modulus ofelasticity exceeding 600.000 kp/cm².
 16. A board as claimed in claim 14,characterized in that the adhesive comprises a PVA emulsion adhesivewith a water content of 40-60% at the time of its application.
 17. Aboard as claimed in claim 1, characterized in that the reinforcingthreads are disposed substantially solely at selected edge portions ofthe board.
 18. A board as claimed in claim 14, characterized in that thereinforcing threads are situated closer to the medial plane of the boardthan to the major surfaces thereof, thereby smoothing out possibledifferences in the expansion properties of the board material andreinforcing threads.